1. Field of the Invention
The present invention relates to an impeller structure for pumping devices, such as a water pump for circulating cooling water in water jackets provided in cylinder blocks of an internal combustion engine.
2. Description of the Prior Disclosure
Conventionally, a water pump having a rotating impeller as shown in FIGS. 1 and 2 is well known. Such impeller type water pump is comprised of a pump body 2 fixed at the front end of a cylinder block 1 between the block and a radiator (not shown), a pump impeller 6, a rotating shaft 3 securely connected to the impeller 6, a bearing 4 disposed in the bore of the pump body 2 so as to rotatably receive the shaft 3. One end of the shaft 3 is secured at the center portion of a pulley 5, having a driven connection with an engine crankshaft, so as to transmit torque to the impeller 6. The impeller 6 is fixed on the other end of the shaft 3. In this construction, the impeller rotates about the axis of the shaft 3 in a direction shown in the arrow D of FIG. 2, with the result that fluid for engine cooling, for example water, is fed from a pump inlet A to a pump outlet B.
The above described conventional impeller 6 arrangement will be described in detail with reference to FIG. 2. The impeller 6 is comprised of a hub section 7 for mounting the impeller body at the other end of the shaft 3 and a plurality of vanes 8 extending from the outer periphery of the hub section in a substantially radial direction of the shaft. As clearly seen in FIG. 2, the pressure surface of each of the vanes 8 is formed in such a manner that the pressure surface is moderately curved from the innermost end of the vane to the outermost end of the vane, downstream in the rotational direction of the impeller 6. As seen in FIG. 2, the pressure surface of the vane 8 is formed convex. Such a conventional impeller has an outlet angle .beta..sub.2 defined between a tangential line in a circumferential direction of the impeller 6 and another tangential line of the pressure surface at the outermost end of the vane 8. As seen in FIG. 2, an angle defined between the above mentioned two tangential lines is formed in a substantially same manner at any pressure surface of the vane 8, that is, at a whole range from the innermost end of the vane to the outermost end of the vane. In a conventional water pump impeller, the outlet angle .beta..sub.2 is generally set to a value of approximately 60.degree..
However, the aforementioned conventional impeller having vanes, each having an outlet angle of approximately 60.degree. provides a relatively low pump efficiency as defined by a ratio of fluid power to pump input power, for example 30%. This is because the input angle, i.e., the approach angle to the fluid is a relatively large angle, such as 60.degree.. This results in energy loss of rotation of the impeller. If the approach angle is decreased simply, characteristic for catching fluid mass may also be lowered. Therefore, it is desirable to enhance catching characteristic for fluid mass without lowering rotational energy loss of the impeller for providing a high pump efficiency.